In the field of composite solid propellants for various rocket motors, a continuing desire is to increase the burning rate. An increase in the burning rate of a solid propellant leads to an increase in the mass flux of combustion gases, hence an increase in thrust. Accordingly, it becomes possible to increase the launching velocity of the rocket or, when there is no need of increasing the launching velocity, to reduce the burning surface area. In the latter case, the loading efficiency (mass fraction of propellant) may be increased. Thus, solid propellants of increased burning rates are the basis of solid propellant rocket motors of relatively small size but relatively high thrust and will certainly contribute to broadening of the applicability of end-burning rocket motors.
Typically, the burning rate of a composite solid propellant is increased with minimum effect on other properties by using an additive that catalyzes the reaction between the oxidizer and the binder (fuel) in the propellant. Until now, various metal oxide powders have been proposed as the burning rate increasing additive or catalyst, but most of them have proved to be impractical because they promote degradation of conventional rubber binders. The most useful binders contain a substantial concentration of polybutadiene, an unsaturated hydrocarbon that provides elasticity. The olefinic unsaturation is very vulnerable to attack by atmospheric oxygen, particularly when catalyzed by traces of most elements of the first transition series of the periodic chart. Thus, propellants formulated with oxides of these metallic elements suffer hardening and embrittlement in storage and become unfit for use.
The most practical choice among the hitherto proposed metal oxide catalysts has been ferric oxide, either hydrous (FeOOH) as described in U.S. Pat. No. 4,424,085; Anchor FY-842.TM. FeOOH (Toho Ganryo Kogyo Co. Ltd.); and Mapico Yellow 300.TM. FeOOH (Cities Service Co., Columbian Division Citgo) or anhydrous (Fe.sub.2 O.sub.3). The major limitation of ferric oxide is that the catalytic efficiency diminishes rapidly as its concentration is increased. Moreover, acceptable rheology of the uncured propellant and tensile properties after cure impose a strict upper limit on the allowable concentration of solid ingredients. Hence, ferric oxide, which provides a negligible contribution to impulse, must be added at the expense of ballistically valuable solid ingredients, typically ammonium perchlorate oxidizer or powdered aluminum fuel.
To circumvent the aforementioned solids loading limitation, it has been proposed to use liquid organoiron compounds (e.g., alkylferrocene derivatives) (see U.S. Pat. No. 4,120,709) as catalysts for increasing burning rate. These substances offer acceptable fuel value and may be substituted for a fraction of the organic binder. Thus, they may be used at relatively high concentration with relatively little degradation of rheology, tensile properties or delivered impulse. Unfortunately, these catalysts suffer several important limitations. They greatly depress the autoignition temperature of a propellant and thereby increase the hazard of accidental ignition by friction, impact, or any other source of heat. They also migrate readily from the propellant into inert organic substrates such as liners and insulators. Finally, they can impair aging stability at elevated temperature in air. In addition, they are relatively expensive.
Accordingly, there has been a constant search in this field of art for propellant catalysts having improved properties.